Growth, condition, reproductive potential, and mortality of bay scallops, Argopecten irradians, in response to eutrophic-driven changes in food resources

Abstract

Anthropogenic nutrient enrichment of coastal waters is changing the habitat and food resources of bay scallops, Argopecten irradians. As land-derived nitrogen (N) enters estuaries, phytoplankton abundance, particulate organic matter, and nitrogen content of seston may increase, providing a higher quantity and quality of food. Understanding these changes is important for monitoring declining populations or developing field aquaculture systems. To examine if changes in food resources due to nutrient enrichment will affect growth, condition, reproductive potential, and mortality, we conducted a field experiment in seven estuaries each having a different land-derived N load. We placed juvenile bay scallops within these estuaries for approximately 12 weeks while monitoring food quantity and quality. Stable isotopic signatures suggested that scallops assimilated food from the specific estuaries in which we placed them. Growth rates were relatively high and did not increase with higher phytoplankton concentrations, suggesting that at the densities we deployed, ambient phytoplankton concentrations were in excess of consumption ability. Growth rates decreased at sites with lower salinities, and where high densities of competitors (barnacles and slipper shells) fortuitously settled on the scallops. Condition index significantly increased with higher growth rates. Gonad index and mortality were not related to food resources, but mortality increased with lower salinities. Land-derived N load seems unlikely to directly alter condition, reproductive potential, or mortality. These results suggest that estuaries undergoing anthropogenic nutrient additions may provide food concentrations above the maximum ration assimilable, resulting in high bay scallop growth rates.